The long-term goal of this project is to further our understanding of how the auditory system forms pitch percepts and how those percepts are used to separate sounds coming from different sources. The project has three main aims. The first aim is to use a combination of psychophysical experiments and quantitative modeling to investigate how pitch is coded in the normal and impaired human auditory system. Experiments will investigate the ability of normal-hearing (NH) and cochlearly hearing-impaired (HI) listeners to hear out, or resolve, individual harmonic components within a complex, and relate this to overall accuracy in pitch coding and to independent measures of frequency selectivity. It is hypothesized that the pitch produced by resolved harmonics is qualitatively different from that produced by unresolved harmonics alone and that HI listeners with poorer frequency selectivity may often have to rely on the latter, less accurate, form of pitch coding. The second aim is to use similar methods to test the limits of pitch perception in more complex situations. These experiments will test NH and HI listeners' ability to hear two pitches at once and to hear out one pitch in the presence of a competing hannonic sound. It may be that these abilities rely on the presence of at least some resolved harmonics; if so, HI listeners may often not be in a position to hear two pitches, or to hear out one in the presence of another. The third aim is to evaluate separately the influence of envelope and fine-structure coding (both of which are thought to play important but different roles in pitch perception) in our ability to perceptually separate different acoustic sources. This approach is more applied and attempts to quantify the information necessary to transmit speech in complex backgrounds, such as a competing talker or a fluctuating noise. The results may have significant implications for the design of cochlear-implant processors and for signal-processing algorithms for hearing aids.